In general, communication systems can be separated into two types: analog and digital. Each type of communication system comprises at least a plurality of communication units and a limited number of communication channels transceived by repeaters. Analog and digital communication systems differ in their information representations, and hence, the manner in which information (particularly audio information) is transceived.
Within an analog transmitter, an analog waveform is used to directly modulate an RF (radio frequency) channel. When the modulated RF channel is received by an analog receiver, the analog waveform is recovered, with the possible addition of noise incurred by the RF channel itself. Within a digital transmitter, an analog waveform is typically sampled to create a continuous stream of digital data, which is then used to modulate an RF channel. When the digitally modulated RF channel is received by a digital receiver, the digital data is recovered. The noise inherent to the RF channel is reflected as errors in the recovered digital data, and thus, is present when the analog waveform is reconstructed from the recovered digital data.
To substantially eliminate noise from being reconstructed, error correction codes are used. Error correction codes provide protection against errors by creating redundancy for the data being transmitted. This redundancy takes the form of additional bits appended to the data, often called parity bits. Prior to transmission over a communication channel, the data is passed through a known encoding function which generates a unique code word, composed of the original data and its parity bits. The code word is then transmitted, via the communication channel, to a receiver. The receiver passes the received code word through a decoding function which will use the parity bits to determine if any errors have occurred in the digital data. Depending on the encoding/decoding function used, a finite number of errors within the data can be detected and corrected, leading to a reduced distortion in the reconstructed analog waveform. By detecting the number of errors which occur during a finite period of time, a bit error rate (BER) can be calculated. The error correction will cease correcting errors only when too many errors have occurred in the data, or alternately stated, when the BER is too high.
Thus, an audio message transmitted through a given RF channel in a digital error-corrected system can result in higher audio quality than the same message transmitted through the same RF channel in an analog system. Since the noise added to a message by an RF channel is directly proportional to the distance between the transmitter and the receiver, a digital error-corrected communication system can offer more uniform audio quality (i.e. less noise) over a larger portion of a given transmitter's coverage area than an analog system. However, when a digital communication unit approaches the outer limits of the transmitter's coverage area, the errors will affect the audio quality with rapidly increasing degradation. The extent of coverage improvement and the point at which audio quality rapidly degrades are both dependent upon the error correction capability of the particular error correction code used. Specifically, as the capabilities of the error correction code are increased, both the extent of coverage improvement and the point at which audio quality rapidly degrades are pushed closer to the outer limits of the coverage area.
While the provision of increased audio quality over a larger portion of a coverage area is useful in itself, it does create a problem in "fringe" coverage area determination by the user. A fringe coverage area can be defined as that portion of a transmitter's coverage area in which the increased presence of received, channel-induced, noise indicates the approaching outer limit of the coverage area. Typically, these fringe areas occur at the most distant regions relative to the transmitter's location, or the coverage area where the received signal is the weakest. Frequently, users of analog systems listen to the amount of background noise in received audio to judge the strength of the received signal, and consequently, where they are relative to the outer limits of the coverage area. A more direct measure of this can be obtained through the calculation of received signal strength information (RSSI). The use of RSSI is known for out-of-range indications in paging systems. RSSI is also used within cellular telephone systems to determine when a hand-off between cells should occur and to establish visual bar-graphs of relative signal strength as a cellular communication unit roams throughout a given coverage area. While these RSSI-based methods are useful in the indication of a low signal strength condition, current art provides an audible indication that interferes with the message. One example of such current art is U.S. Pat. No. 5,134,708 to Marui et al., granted Jul. 28, 1992, entitled "Radio Telephone Apparatus."
Within an error-corrected digital system, the error correction effectively reduces the perceptible fringe coverage areas by masking the added noise effects of reduced signal strength. This in turn leaves a user less time and distance to recognize the degradation in audio quality and thus take corrective actions which will return them to an area of improved coverage. Therefore a need exists for a method which provides an audible early indication that a user in a digital communication system is approaching the outer limits of a given coverage area, such that the received signal audio quality is not further degraded by the presence of the indicator.